About half of diners find their portions of food so large they can't finish them when they eat out, a new survey says.

The environmentalist group Greeners Action polled 988 people on the streets last month.

Half of the respondents said they could not finish their rice or noodle dishes, while 58 per cent said they were served the same excessive amount of food even after they had asked for smaller portions.

A similar survey in 2009 found that only 16 per cent of those polled said they could finish their meals, as opposed to 13 per cent in 2006.

The green group's executive director Angus Ho Hon-wai said the figures show many Hongkongers are eager to minimise food waste.

He adds inflation has caused restaurants to reduce food portions. Campaigns against food waste have also had an effect.

Since 2006, Greeners Action has been co-operating with 146 restaurants across the city to give diners a HK$1 discount if they ask for less rice.

Ho said the campaign did not only help eateries reduce the amount of leftovers; it also helped decrease restaurants' operating costs by up to 20 per cent.

The group has also urged the government to focus on cutting waste at source and to consider building more organic waste-treatment plants.

A food waste recycling centre in Siu Ho Wan on Lantau Island will open in 2015. But it will only have the capacity to handle 200 tonnes of food waste daily. About 3,000 tonnes are produced each day.